A dummy battery is a battery that is just an electrical conductor, not a real battery.

Let me put it to you this way, you've got a blaster with room for four AA batteries. Those Duracells you've been using all this time are 1.5 volt each. Do the math, and you have 6 volts of power.

But lets' say you splurge and get some 3.7 volt batteries from eBay. Well, putting four 3.7 volt batteries in yer blaster is going to generate 14.8 volts, do the math. That's WAY too much for the stock motors and the thermistor cut-offs that come installed with yer blaster. You'll burn your motors out it out in a month, and the thermistors will kick in and limit you to 10 minutes play time.

Enter the Dummy Battery. You put two 3.7 volt batteries, and two dummy batteries that are basically just electrical conductors in your blaster, and you've got 7.2 volts instead of 6 volts. You see, two times 3.7, plus a couple dummy batteries that add zero volts, equals 7.2 volts. Blaster is happy, user is happy, everyone is happy.